Also, check your modes. Some modes have gray scale as part of the settings - sleep mode, for one. You might have the color correction icon on your quick panel and accidentally toggled it on (it looks like an eyedropper).
If it's only icons that are grayscale, it's possible that the current Mode is set to limit what apps you can open. You can change, view, and edit settings related to Modes in the Modes & Routines section of Settings.
Is it the apps, app icons or your entire screen? If it's the icons only try selecting a free icon pack from Galaxy themes to see if that changes anything.
